The heart of a computer is an assembly that is referred to as a magnetic disk drive. The magnetic disk drive includes a rotating magnetic disk, write and read heads that are suspended by a suspension arm adjacent to a surface of a rotating magnetic disk and an actuator that swings the suspension arm to place the read and write heads over selected circular tracks on the rotating disk. The read and write heads are directly located on a slider that has an air bearing surface (ABS). The suspension arm biases the slider into contact with the surface of the disk when the disk is not rotating but, when the disk rotates, air is swirled by the rotating disk. When the slider rides on the air bearing, the write and read heads are employed for writing magnetic impressions to and reading magnetic impressions from the rotating disk. The read and write heads are connected to processing circuitry that operates according to a computer program to implement the writing and reading functions.
The write head includes a coil layer embedded in first, second and third insulation layers (insulation stack), the insulation stack being sandwiched between first and second pole piece layers. A gap is formed between the first and second pole piece layers by a gap layer at an air bearing surface (ABS) of the write head and the pole piece layers are connected at a back gap. Current conducted to the coil layer induces a magnetic flux in the pole pieces which causes a magnetic field to fringe out at a write gap at the ABS for the purpose of writing the aforementioned magnetic impressions in tracks on the moving media, such as in circular tracks on the aforementioned rotating disk.
In recent read head designs a spin valve sensor, also referred to as a giant magnetoresistive (GMR) sensor, has been employed for sensing magnetic fields from the rotating magnetic disk. The sensor includes a nonmagnetic conductive layer, hereinafter referred to as a spacer layer, sandwiched between first and second ferromagnetic layers, hereinafter referred to as a pinned layer and a free layer. First and second leads are connected to the spin valve sensor for conducting a sense current therethrough. The magnetization of the pinned layer is pinned perpendicular to the air bearing surface (ABS) and the magnetic moment of the free layer is located parallel to the ABS, but free to rotate in response to external magnetic fields. The magnetization of the pinned layer is typically pinned by exchange coupling with an antiferromagnetic layer.
The thickness of the spacer layer is chosen to be less than the mean free path of conduction electrons through the sensor. With this arrangement, a portion of the conduction electrons is scattered by the interfaces of the spacer layer with each of the pinned and free layers. When the magnetizations of the pinned and free layers are parallel with respect to one another, scattering is minimal and when the magnetizations of the pinned and free layer are antiparallel, scattering is maximized. Changes in scattering alter the resistance of the spin valve sensor in proportion to cos θ, where θ is the angle between the magnetizations of the pinned and free layers. In a read mode the resistance of the spin valve sensor changes proportionally to the magnitudes of the magnetic fields from the rotating disk. When a sense current is conducted through the spin valve sensor, resistance changes cause potential changes that are detected and processed as playback signals.
A spin valve sensor is characterized by a magnetoresistive (MR) coefficient that is substantially higher than the MR coefficient of an anisotropic magnetoresistive (AMR) sensor. For this reason a spin valve sensor is sometimes referred to as a giant magnetoresistive (GMR) sensor. When a spin valve sensor employs a single pinned layer it is referred to as a simple spin valve. When a spin valve employs an antiparallel (AP) pinned layer it is referred to as an AP pinned spin valve. An AP spin valve includes first and second magnetic layers separated by a thin non-magnetic coupling layer such as Ru. The thickness of the spacer layer is chosen so as to antiparallel couple the magnetizations of the ferromagnetic layers of the pinned layer. A spin valve is also known as a top or bottom spin valve depending upon whether the pinning layer is at the top (formed after the free layer) or at the bottom (before the free layer).
The spin valve sensor is located between first and second nonmagnetic electrically insulating read gap layers and the first and second read gap layers are located between ferromagnetic first and second shield layers. In a merged magnetic head a single ferromagnetic layer functions as the second shield layer of the read head and as the first pole piece layer of the write head. In a piggyback head the second shield layer and the first pole piece layer are separate layers.
Traditionally, the magnetization of a pinned layer has been fixed by exchange coupling one of the ferromagnetic layers (AP1) with a layer of antiferromagnetic material such as PtMn. While an antiferromagnetic material such as PtMn does not in and of itself have a magnetization, when exchange coupled with a magnetic material, it can strongly pin the magnetization of the ferromagnetic layer.
The ever increasing quest for faster data rates and increased data capacity requires ever decreasing bit lengths, in order to fit more bits onto a given inch of data track. This in turn requires sensor to have decreased sensor thickness (stack height). Unfortunately, in order to pin a pinned layer as described above through exchange coupling with an AFM layer, the AFM layer must be deposited very thick relative to the other layers in the sensor stack. In the drive for decreased sensor thickness such an AFM layer uses an unacceptable amount of gap budget.
In order to overcome this, sensors have recently been designed with self pinned pinned layers. Pinning in a self pinned layer is generally achieved primarily by stress induced anisotropy caused by constructing the pinned layers of a material having a high positive magnetostriction. Compressive forces which inevitably occur in a sensor, in combination with the high positive magnetostriction of the pinned layers causes the magnetization to be pinned in a desired direction perpendicular to the air bearing surface (ABS). A self pinned structure includes first and second layers of ferromagnetic material that are antiparallel coupled across a coupling layer such as Ru, similar to the conventional AP coupled pinned layer described above. In order to increase pinning strength, in a self pinned layer structure the magnetic thicknesses of the two ferromagnetic layers (AP1 and AP2) are substantially equal (ie. δm=0).
Although self pinned structures have shown promise for decreasing stack height of a sensor, the can be prone to amplitude flipping. Amplitude flipping occurs when the directions of magnetization of the pinned layer flip 180 degrees. This renders the head unusable. As discussed above, self pinned sensors rely on stress induced anisotropy of magnetostricive materials used in the pinned layers. The magnetostriction of a material is dependent on temperature. The magnetosrictive properties of the materials making up the pinned layers in a self pinned head greatly decrease or cease altogether at higher temperatures. During and event such as a head disk contact, or an electrostatic discharge (ESD), temperatures can rise to the point that the materials making up the pinned layer momentarily cease to be magnetostrictive. In addition, mechanical strains on the head during an ESD event or head disk interface can cause the compressive stresses to momentarily cease. This momentary loss of magnetostriction and/or compressive stress can momentarily eliminate the anisostropy that maintains pinning of the self-pinned layers. This can allow the orientation of the magnetic moments to flip directions 180 degrees, rendering the head useless.
As can be seen from the above, there is a strong felt need for a means for stabilizing pinning of a self pinned sensor in order to take advantage of the decreased thickness provided by the use of self pinned layers while avoiding unacceptable amplitude flipping. Such a stabilizing mechanism would provide such self-pinned heads with sufficient robustness to render them practical for consumer and commercial application.